1. Technical Field
The present invention relates to a tailstock control device which supports an object to be machined by controlling a servo motor which drives a driving system of a tailstock having a spring member in forward and backward directions, to cause a tailstock center connected to the tailstock to contact a center hole of the object to be machined.
2. Related Art
Conventionally, driving of a tailstock which supports an object to be machined on a numerical control lathe has been largely by hydraulic driving. In recent years, however, electric tailstocks which are driven and controlled by a servo motor are employed.
One reason why an electric tailstock has been employed is that with the servo motor, the supporting thrust of the object to be machined can be freely controlled by controlling the drive torque. There are also other reasons such as that the non-operational time, such as the thrust change during setup of high-mix low-volume workpiece, can be reduced by controlling the support thrust, and that the energy consumption of the tool can be reduced by removing the hydraulic unit.
FIG. 2 shows an example of a mechanism of an electric tailstock which is driven and controlled by a servo motor. In this mechanism, a feed screw 106 connected to a servo motor 9 rotates with the driving of the servo motor 9. With this rotation, a tailstock 102 which is threaded and connected to the feed screw 106 is driven and controlled. As a result, a tailstock center 101 attached to the tailstock 102 is mated with the center hole of an object 100 to be machined, and the object 100 to be machined is supported. In order to maintain the support thrust for supporting the object to be machined even after the power of the motor is switched off, a spring 104 is typically provided at the rear side of the tailstock center 101.
In this mechanism, the drive torque (torque limit value Tm) of the servo motor required for generating a thrust (support thrust) necessary for reliably supporting the object to be machined is calculated based on the support thrust. A transmission mechanism exists in the driving system. Because of this, in general, the limit torque value Tm is calculated in consideration of the transmission efficiency of the transmission mechanism and subtracting a torque loss due to the driving system from the support thrust. In other words, in the related art, the torque limit value Tm of the servo motor necessary for generating the support thrust is calculated based on the support thrust and the transmission efficiency of the driving system. Next, the related art will be described with reference to a block diagram of FIG. 7.
A tailstock support thrust Fa which is set in advance in a tailstock thrust setting unit 1 is input to a torque instruction converter unit 3. Tailstock parameters N which indicate the torque loss of the driving system such as a gear ratio of the driving system which is set in a tailstock parameter setting unit 2, a tailstock sliding surface resistance, frictional torque of the feed screw shaft receiving unit, and efficiency of a ball screw are also input to the torque instruction converter unit 3. The torque instruction converter unit 3 calculates, as the torque limit value Tm, a torque value of the servo motor necessary for generating the support thrust Fa, based on the support thrust Fa and the transmission parameters N.
A tailstock instruction unit 5 generates a tailstock instruction by a manual operation or by a machining program (neither of which is shown), and activates a tailstock control unit 6. The activated tailstock control unit 6 controls the torque of the servo motor 11 through a servo control unit 7 and a power amplifier unit 10, based on parameters such as a tailstock movement velocity Vs which are output from a tailstock velocity setting unit 4. The servo control unit 7 calculates an instruction torque value Tr in response to the instruction from the tailstock control unit 6 and outputs the instruction torque value to the power amplifier unit 10. The power amplifier unit 10 supplies power corresponding to the instruction torque value Tr to the servo motor 11. At this point, the tailstock control unit 6 stores the torque limit value Tm converted by the torque instruction converter unit 3 in a torque instruction value storage unit 8, and transmits the torque limit value Tm to the servo control unit 7. The servo control unit 7 executes the torque limit control of the servo motor 11 with the upper limit at the torque limit value Tm.
With the above-described structure, the servo motor 11 which drives the tailstock is driven and controlled, and the object to be machined is supported by the tailstock center provided on the tailstock mating with the center hole of the object to the machined. The servo control unit 7 stops the feeding of the tailstock when the instruction torque value Tr (output torque value of the servo motor) reaches the limit torque value Tm. With this process, the object to be machined is supported with a predetermined support thrust.
According to the related art described above, the limit torque value Tm of the servo motor 11 is calculated in consideration of parameters such as the tailstock sliding surface, frictional torque at the feed screw shaft receiving unit, and efficiency of the ball screw, in addition to the tailstock support thrust Fa which is set in advance. Because of this structure, the object to be machined can be supported with the predetermined support thrust Fa. However, in the supporting of the object to be machined, during a period from the time when the feeding operation of the tailstock is stopped when the drive torque Tr reaches the torque limit value Tm to the time when the tailstock actually stops, a slight movement distance is created due to the inertial force, and an excessive thrust is caused. In addition, in a tailstock in which a spring mechanism is built-in to the driving system, the spring is compressed by the movement distance until the tailstock is stopped, and an excessive thrust would be applied to the object to be machined. In addition, there has been a problem in that when the tailstock movement velocity is changed, the movement distance until the tailstock stops is also changed, and variation occurs in the support thrust.